In philosophy terms the difference between reductionism and positivism

is that reductionism is reductionism is a philosophical position which holds that a complex system is nothing but the sum of its parts, and that an account of it can be reduced to accounts of individual constituents. This can be said of objects, phenomena, explanation, theories, and meanings. Reductionism strongly reflects a certain perspective on causality. In a reductionist framework, the phenomena that can be explained completely in terms of relations between other more fundamental phenomena, are called "epiphenomena". Often there is an implication that the epiphenomenon exerts no causal agency on the fundamental phenomena that explain it. Reductionism does not preclude the existence of what might be called "emergent phenomena", but it does imply the ability to understand those phenomena completely in terms of the processes from which they are composed while positivism is a doctrine that states that the only authentic knowledge is scientific knowledge, and that such knowledge can only come from positive affirmation of theories through strict scientific method, refusing every form of metaphysics.

As nouns the difference between reductionism and positivism

is that reductionism is an approach to studying complex systems or ideas by reducing them to a set of simpler components while positivism is a doctrine that states that the only authentic knowledge is scientific knowledge, and that such knowledge can only come from positive affirmation of theories through strict scientific method, refusing every form of metaphysics.